A big part of people having lots of kids was infant mortality, the need for physical labour (farming) and lack of old age security (kids take care of parents).

A big part of people having lots of kids was infant mortality, the need for physical labour (farming) and lack of old age security (kids take care of parents). All 3 of these issues are mostly gone so now parents try to focus resources into ensuring their children are as successful as possible instead of as numerous as possible.